P-T conditions in the South Wales Coalfield: Evidence from coexisting hydrocarbon and aqueous fluid inclusions

Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97-212 degrees C (mean 143 degrees C). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in the range 35-78 degrees C (mean 54 degrees C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a P-T estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperatures between 130 and 160 degrees C, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.